1. Field of the Invention
This invention relates to wavelength-tunable, compact, modular and efficient sources of high-power ultrashort laser pulses, which are an essential ingredient in the commercial use of ultrafast laser technology.
2. Description of the Related Art
Fiber lasers have long been known to provide an effective medium for the generation of ultrashort pulses, though so far such systems have mainly been based on chirped pulse amplification using chirped Bragg gratings, with limited options for wavelength tunability and limitations in the minimal achievable pulse width (A. Galvanauskas and M. E. Fermann, ‘Optical Pulse Amplification Using Chirped Bragg Gratings,’ U.S. Pat. No. 5,499,134). Chirped Bragg gratings have indeed been developed into widely available devices, and the chirp inside the Bragg gratings can be designed to be linear or even nonlinear to compensate any order of dispersion in a chirped pulse amplification system (A. Galvanauskas et al., ‘Hybrid Short-Pulse Amplifiers with Phase-Mismatch Compensated Pulse Stretchers and Compressors’, U.S. Pat. No. 5,847,863), which is important for the generation of bandwidth limited pulses, i.e., the shortest possible pulses for a given spectral pulse bandwidth.
To maximize the power and energy limitations of optical fibers, the use of chirped pulse amplification is clearly desirable, though at the same time the demands for system integration (Bragg gratings need to be operated in reflection rather than in transmission to provide the highest possible dispersion) may render the use of such standard chirped pulse amplification systems impractical. As an alternative to chirped pulse amplification, the amplification of high-power pulses in multi-mode fiber amplifiers has been suggested (M. E. Fermann and D. Harter, ‘Single-mode Amplifiers and Compressors Based on Multi-mode Optical Fibers’, U.S. Pat. No. 5,818,630). As yet another alternative to chirped pulse amplification the use of soliton Raman compression in fiber amplifiers, or, generally, the use of pulse compression inside nonlinear fiber amplifiers was proposed (M. E. Fermann, A. Galvanauskas and D. Harter, ‘Apparatus and Method for the Generation of High-power Femtosecond Pulses from a Fiber Amplifier’, U.S. Pat. No. 5,880,877).
Clearly the use of multi-mode fibers can be combined with chirped pulse amplification and soliton Raman compression to further improve the performance of such systems. However, to date no methods for controlling the pulse-shape for a further optimization of the overall system performance have been described. Equally, the use of self-phase modulation in the stretcher part of such chirped pulse amplification systems has not been suggested.
Moreover, as a compromise between system compactness and high-energy capability, the use of a fiber dispersive delay line in conjunction with a bulk optic compressor can be advantageous, providing at least partial integration of a high-energy fiber laser system [M. E. Fermann A. Galvanauskas and D. Harter: ‘All fiber source of 100 nJ sub-picosecond pulses’, Appl. Phys. Lett., vol. 64, 1994, pp. 1315–1317]. However, to date no effective methods for controlling higher-order 3rd and 4th order dispersion in such stretcher and compressor combinations for the re-compression of the pulses to near their bandwidth limit have been developed.
As an alternative to chirped pulse amplification, it was also previously suggested that efficient pulse compression can be obtained by using high-gain positive dispersion (non-soliton supporting) silica-based single-mode erbium amplifiers in combination with bulk prism compressors (K. Tamura and M. Nakazawa, “Pulse Compression by Nonlinear Pulse Evolution with Reduced Optical Wave Breaking in Erbium-Doped Fiber Amplifiers,” Opt. Lett., Vol. 21, p. 68 (1996)). However, the use of this technique in conjunction with silica-based erbium amplifiers is problematic, because the requirement for positive dispersion limits the fiber core size to around 5 μm, otherwise, negative material dispersion dominates over positive waveguide dispersion, producing an overall negative fiber dispersion. Equally, silica-based multi-mode fibers have negative dispersion at erbium amplifier wavelengths, preventing their use in efficient pulse compression. Thus, the limited core size of positive dispersion erbium amplifiers greatly reduces the achievable pulse energy.
Moreover, it was not shown by Tamura et al. how to generate additional spectral broadening and pulse amplification after the one erbium amplifier. Equally, it was not taught by Tamura et al. how to optimize the performance of the prism pulse compressor to compensate for the dispersion of the erbium amplifier.
As another alternative to chirped pulse amplification, the use of a non-amplifying optical fiber in conjunction with a bulk grating compressor was suggested (D. Grischkowsky et al. and J. Kafka et al., U.S. Pat. No. 4,750,809). However, since there is no gain in such a system, high pulse energies have to be coupled into the nonlinear optical element in order to obtain a high output power, greatly reducing the peak power capability of the system. Moreover, no means for compensating for higher-order dispersion in such an optical arrangement was discussed, greatly limiting the practicability of this approach. In addition, without control of the pulse shape at the input to such a system, spectral broadening with a linear chirp can only be obtained for very limited input powers. Control of the input pulse shape was not discussed by Kafka et al. Equally, to obtain the shortest possible pulses in conjunction with a bulk grating compressor, the control of 2nd and 3rd order dispersion in such a nonlinear optical element is required, which was also not discussed by Kafka et al.
Compensation for chromatic dispersion in a (low-power) lightwave signal using the chromatic dispersion in another (dispersion-compensating) waveguide element was introduced to optimize the performance of telecommunication systems (C. D. Poole, ‘Apparatus of compensating chromatic dispersion in optical fibers’, U.S. Pat. No. 5,185,827). However, for high-power pulse sources, self-phase modulation introduced by a dispersion-compensating waveguide element prevents their effective use. Moreover, the system discussed by Poole only operates in conjunction with mode-converters and/or rare-earth-doped fiber for either selectively absorbing a higher-order spatial mode in the dispersion-compensating waveguide element or selectively amplifying the fundamental mode in the dispersion-compensating waveguide element. No means were taught for compensating for the dispersion of high-power optical pulses in the presence of self-phase modulation, and no means of implementing a dispersion-compensating waveguide without mode-converters were suggested.
As an alternative to the use of mode-converters and higher-order modes, fibers with W-style refractive index profiles are known (B. J. Ainslie and C. R. Day, ‘A review of single-mode fibers with modified dispersion characteristics’; J. Lightwave Techn., vol. LT-4, No. 8, pp. 967–979, 1988). However, the use of such fiber designs in high-power fiber chirped pulse amplification systems has not been discussed.
To maximize the efficiency of ultrafast fiber amplifiers, the use of Yb fiber amplifiers has been suggested (D. T. Walton, J. Nees and G. Mourou, “Broad-bandwidth pulse amplification to the 10 μJ level in an ytterbium-doped germanosilicate fiber,” Opt. Lett., vol. 21, no. 14, pp. 1061 (1996)), though the work by Walton et al., employed an Argon-laser pumped Ti:sapphire laser for excitation of the Yb-doped fibers as well as a modelocked Ti:sapphire laser as a source of signal pulses, which is extremely inefficient and clearly incompatible with a compact set-up. Moreover, no means for controlling the phase of the optical pulses in the amplification process were suggested, i.e., 100 fs pulses from the Ti:sapphire laser were directly coupled to the Yb amplifier through a 1.6 km long single-mode fiber dispersive delay line, which produces large phase distortions due to higher-order dispersion, greatly limiting the applicability of the system to ultrafast applications. Rather, to induce high-quality high-power parabolic pulse formation inside the Yb amplifier, seed pulses in the range from 200–400 fs would be preferable for an Yb amplifier length of a few meters. The use of a single-mode Yb-doped fiber amplifier by Walton et al. further greatly limited the energy and power limits of the Yb amplifier. The use of a multi-mode Yb-doped fiber was suggested in U.S. application Ser. No. 09/317,221, the contents of which are hereby incorporated herein by reference, though a compact ultrashort pulse source compatible with Yb amplifiers remained elusive.
A widely tunable pulsed Yb-fiber laser was recently described incorporating an active optical modulation scheme (J. Porta et al., ‘Environmentally stable picosecond ytterbium fiber laser with a broad tuning range’, Opt. Lett., vol. 23, pp. 615–617 (1998). Though this fiber laser offered a tuning range approximately within the gain bandwidth of Yb, application of the laser to ultrafast optics is limited due to the relatively long pulses generated by the laser. Generally, actively modelocked lasers produce longer pulses than passively modelocked lasers, and in this present case the generated pulse bandwidth was only 0.25 nm with a minimal pulse width of 5 ps.
Widely wavelength-tunable fiber laser sources were recently described using Raman-shifting in conjunction with frequency-conversion in a nonlinear crystal. (See M. E. Fermann et al., U.S. Pat. No. 5,880,877 and N. Nizhizawa and T. Goto, “Simultaneous Generation of Wavelength Tunable Two-Colored Femtosecond Soliton Pulses Using Optical Fibers,” Photonics Techn. Lett., vol. 11, no. 4, pp 421–423). Essentially spatially invariant fiber Raman shifters were suggested, resulting in limited wavelength tunability of 300–400 nm (see Nizhizawa et al.). Moreover, no method was known for minimizing the noise of such a highly nonlinear system based on the successive application of Raman shifting and nonlinear frequency conversion in a nonlinear optical crystal. Further, the system described by Nizhizawa et al. relied on a relatively complex low power polarization controlled erbium fiber oscillator amplified in an additional polarization controlled erbium fiber amplifier for seeding the Raman shifter. Moreover, no method was described that allowed Raman-shifting of the frequency-doubled output from an Er fiber laser.
A Raman shifter seeded directly with the pulses from a high-power fiber oscillator or the frequency-converted pulses from a high-power fiber oscillator would clearly be preferable. Such fiber oscillators were recently described using multi-mode optical fibers (M. E. Fermann, ‘Technique for mode-locking of multi-mode fibers and the construction of compact high-power fiber laser pulse sources’, U.S. Ser. No. 09/199,728). However, to date no methods for frequency-converting such oscillators with the subsequent use of Raman-shifting have been demonstrated.